This invention relates to an improved process for the preparation of urea from ammonia and carbon dioxide.
If ammonia and carbon dioxide are introduced into a urea synthesis zone at a suitable elevated pressure (for instance 125-350 atmospheres) and temperature (for instance 170.degree.-250.degree. C.), ammonium carbamate is formed according to the reaction: EQU 2 NH.sub.3 +CO.sub.2 .fwdarw.H.sub.2 N--CO--ONH.sub.4
which is in turn converted into urea by dehydration according to the equilibrium reaction: EQU H.sub.2 N--CO--ONH.sub.4 .revreaction.H.sub.2 N--CO--NH.sub.2 +H.sub.2 O.
The degree to which this latter conversion proceeds is dependent upon the temperature and the amount of excess ammonia and water present in the synthesis zone.
The urea synthesis effluent thereby obtained consists essentially of urea, water, nonconverted ammonium carbamate and excess ammonia. The ammonium carbamate and excess ammonia are thereupon removed from the solution and most generally they are recycled to the synthesis zone.
The synthesis zone may be a single zone in which both the ammonium carbamate and urea forming reactions proceed concurrently, or it may be divided into two separate zones for the formation, respectively, of ammonium carbamate and urea.
One process often applied for the preparation of urea is described in European Chemical News, Urea Supplement of Jan. 17, 1969, pages 17-20. In this process, the urea synthesis solution, formed at an elevated pressure and temperature, is subjected to a stripping treatment at the synthesis pressure by simultaneously heating and countercurrently contacting the solution with gaseous carbon dioxide. This results in the decomposition of a major portion of the ammonium carbamate present in the synthesis solution into ammonia and carbon dioxide, and the removal of these decomposition products from the residual urea solution as a gas mixture, together with excess ammonia, a minor quantity of water vapor, and the carbon dioxide used for stripping. This stripping treatment can be effected not only with carbon dioxide as described in the publication, but also with gaseous ammonia, an inert gas, or a mixture of any two or more of these gases (see, for instance, U.S. patent application Ser. No. 312,662 of P. Kaasenbrood).
A major portion of a gas mixture obtained in this stripping treatment is fed to a first condensation zone wherein it is condensed and absorbed into an aqueous solution originating from a subsequent treatment of the urea-containing solution. Thereafter, both the ammonium carbamate solution thus formed and the non-condensed gas mixture are recycled to the urea synthesis zone wherein the heat required for the conversion of ammonium carbamate into urea is obtained by further condensation of the gas mixture into ammonium carbamate.
The product urea solution from the stripping zone, still containing residual ammonium carbamate, is subsequently expanded to a low pressure (for instance 3-6 bar) and heated by means of steam so as to decompose a further amount of ammonium carbamate and remove the decomposition products, together with an amount of water vapor. The gas mixture obtained from this ammonium carbamate decomposition zone is condensed in a second condensation zone operated at a relatively low pressure wherein it is absorbed into an aqueous solution absorption agent to form a dilute ammonium carbamate solution. This dilute ammonium carbamate solution is pumped back up to the pressure of the high-pressure part of the urea synthesis and ultimately recycled into the urea synthesis zone.
The remaining product urea solution removed from the decomposition zone is further reduced in pressured and worked up to a concentrated aqueous urea solution, or it is further processed into solid urea. In so doing, water is removed from the urea solution by evaporation, and the urea melt thus obtained is processed into granules, or the concentrated urea solution is crystallized. The gases obtained from this evaporation or crystallization step contain, in addition to water vapor, an amount of ammonia, carbon dioxide, and entrained urea droplets, all of which is condensed to form process condensate. A portion of this process condensate is used as the aqueous absorption agent for the gas mixture condensed in the second condensation zone. A remaining portion of this process condensate can be treated with high-pressure steam so as to decompose or hydrolyze the urea present into ammonia and carbon dioxide, and to recover the decomposition products, together with the ammonia and carbon dioxide present in the process condensate, by means, for instance, as described in Industrial Wastes, September/October, 1976, pages 44-47.
Inasmuch as the conversion of ammonium carbamate into urea in the urea synthesis zone is an equilibrium reaction in which water is formed, the amount of water recycled, and thus present in the synthesis zone, should be as small as possible. Therefore, it is advantageous to have the recycled ammonium carbamate solution as concentrated as possible. However, to prevent the formation of solid ammonium carbamate in the recycled solution, a certain minimum amount of water must be present, depending upon the temperature, and thus the pressure. This minimum amount of water required in the carbamate solution decreases with increasing temperature, and thus with increasing pressure.
Thus, it is inevitable that some amount of water is introduced into the synthesis zone with the carbamate solution formed in the second condensation zone, which is supplied to the synthesis zone via, for instance, the first condensation zone, which adversely effects the efficiency of the conversion of ammonium carbamate into urea.